In one known apparatus for measuring the amount, that is, the length of yarn withdrawn from the warp beam of a warp knitting machine, a pressure roll engages the warp beam to rotate with the warp beam as the warp beam is rotated. The pressure roll drives directly a tachogenerator which produces a signal corresponding to the speed of the yarns as they are withdrawn from the warp beam, and, thus, the amount of yarns which have been withdrawn from the warp beam. In the apparatus, the tachogenerator includes a digital pulse generator having output pulses which are spaced a distance corresponding to the respective length of delivered yarn. To prevent damage to yarns wound on the warp beam by contact of the pressure roll, the contact pressure roll is normally formed from a relatively soft rubber. The pulses emitted by the pulse generator are compared with a predetermined pulse train to derive from the comparison a measure for controlling the rotation of the warp beam. The pressure roll includes an output shaft acting as a direct drive of the pulse generator.
This type of yarn measuring apparatus in which the soft pressure roll contacts the actual yarn on the warp beam has several drawbacks. The resistance imparted onto the pressure roll by the pulse generator connected directly to the pressure roll shaft may cause slippage of the yarns. If slippage occurs, the generated output pulses will not accurately represent the actual yarn withdrawn from the warp beam. Additionally, slippage creates wear on the pressure roll resulting in inaccurate measured and calculated results because the number of generated output pulses is determined by the number of rotations of the pressure roll. Wear decreases the diameter of the roll resulting in an increased number of pulses per revolution of the roll. Thus, the measured amount of yarn withdrawn from the warp beam will be higher than the actual amount withdrawn when the pressure roll is withdrawn.